test: Add tests for CPU and memory graph pages
test: Add tests for CPU and memory graph pages

This makes sure that the page configures as expected and doesn't crash.

Don't test the innards of the graph again, as they are exactly the same
widgets as the ones already tested further above. But do ensure that
they have a reasonable size.

Reproduces https://bugzilla.redhat.com/show_bug.cgi?id=1792623

Closes #13439

cockpituous/cockpit,deryni/cockpit,garrett/cockpit,moraleslazaro/cockpit,deryni/cockpit,garrett/cockpit,martinpitt/cockpit,deryni/cockpit,mvollmer/cockpit,deryni/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpituous/cockpit,martinpitt/cockpit,mvollmer/cockpit,garrett/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpituous/cockpit,garrett/cockpit,deryni/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,deryni/cockpit,mvollmer/cockpit,cockpituous/cockpit,deryni/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,garrett/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,cockpituous/cockpit,cockpit-project/cockpit